Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods473late end point and a positive determinate error are possible if we usea pH of 11.2. Why is a small amount of the Mg 2+ –EDTA complex added to thebuffer?The titration’s end point is signaled by the indicator calmagite. Theindicator’s end point with Mg 2+ is distinct, but its change in colorwhen titrating Ca 2+ does not provide a good end point. If the sampledoes not contain any Mg 2+ as a source of hardness, then the titration’send point is poorly defined, leading to inaccurate and impreciseresults.Adding a small amount of Mg 2+ –EDTA to the buffer ensures thatthe titrand includes at least some Mg 2+ . Because Ca 2+ forms a strongercomplex with EDTA, it displaces Mg 2+ from the Mg 2+ –EDTAcomplex, freeing the Mg 2+ to bind with the indicator. This displacementis stoichiometric, so the total concentration of hardness cationsremains unchanged. The displacement by EDTA of Mg 2+ from theMg 2+ –indicator complex signals the titration’s end point.3. Why does the procedure specify that the titration take no longer than5 minutes?A time limitation suggests that there is a kinetically controlled interference,possibly arising from a competing chemical reaction. In thiscase the interference is the possible precipitation of CaCO 3 at a pHof 10.9C.4 Quantitative ApplicationsAlthough many quantitative applications of complexation titrimetry havebeen replaced by other analytical methods, a few important applicationscontinue to be relevant. In the section we review the general application ofcomplexation titrimetry with an emphasis on applications from the analysisof water and wastewater. First, however, we discuss the selection and standardizationof complexation titrants.Se l e c t i o n a n d St a n d a r d i z a t i o n o f Ti t r a n t sEDTA is a versatile titrant that can be used to analyze virtually all metalions. Although EDTA is the usual titrant when the titrand is a metal ion, itcannot be used to titrate anions. In the later case, Ag + or Hg 2+ are suitabletitrants.Solutions of EDTA are prepared from its soluble disodium salt,Na 2 H 2 Y•2H 2 O and standardized by titrating against a solution made fromthe primary standard CaCO 3 . Solutions of Ag + and Hg 2+ are prepared usingAgNO 3 and Hg(NO 3 ) 2 , both of which are secondary standards. Standardizationis accomplished by titrating against a solution prepared fromprimary standard grade NaCl.pMgpMgpMg1086420108642086420pH 90 2 4 6 8 10Volume of EDTA (mL)0 2 4 6 8 10Volume of EDTA (mL)10 pH 11early end pointpH 10late end point0 2 4 6 8 10Volume of EDTA (mL)Figure 9.33 Titration curves for 50 mLof 10 –3 M Mg 2+ with 10 –3 M EDTAat pHs 9, 10, and 11 using calmagiteas an indicator. The range of pMg andvolume of EDTA over which the indicatorchanges color is shown for eachtitration curve.